Some automotive vehicle internal combustion engines with fuel injectors utilize a no-return fuel system. In such a system, fuel is typically supplied to one or more fuel rails or distribution manifolds communicating with each fuel injector and no fuel is returned from the fuel rails downstream of any of the fuel injectors. Typically, in a no-return system, fuel is supplied by an electric pump received in the fuel tank to the fuel rail through one or more pressure regulators to supply fuel to the injectors at a substantially constant pressure relative to either the atmosphere or at a substantially constant pressure drop across the operating injectors and hence relative to the absolute pressure in the air intake manifold. Usually, when the engine is running, pressure in the intake manifold downstream of a throttle valve is subatmospheric although in supercharged engines, it may be superatmospheric.
Normally, when a vehicle operator turns off the ignition of an engine, the engine fuel rail remains full of liquid fuel and often a check valve in the system downstream of the pump outlet and upstream of the fuel rail prevents liquid fuel from returning through the pump or otherwise to the fuel tank. Under normal conditions of ambient temperature, all of the fuel in the rail remains in liquid form and upon cranking to restart the engine liquid fuel is immediately available to the engine. However, under certain hot weather or engine overheating conditions, usually referred as "hot soak" conditions, the temperature of fuel in the rail can increase sufficiently for part of the fuel to vapororize and form fuel vapor therein. If it is attempted to start the engine under these hot soak conditions, it will stall, stumble or at best run extremely roughly at a very low speed until all the vaporized fuel in the rail has been consumed and only liquid fuel is again supplied to all of the injectors. Usually, hot soak conditions producing fuel vapor in the rail occur when the engine is shut off although they sometimes occur when the engine is running. Sometimes, the temperature of fuel in the rail and in the surrounding engine compartment can become as high as about 250.degree. F. and even fuel in the tank can reach a temperature as high as about 125.degree.-130.degree. F.